How to edit my second object only?(instead of edit the first and second objects together)
A way to do its is to use bmesh.ops with mirror.
import bpy
import bmesh

# Get the object and its mesh
obj = bpy.context.object
mesh = obj.data

# Give the mesh to a bmesh
bm = bmesh.new()
bm.from_mesh(mesh)

# Call the mirror operation with Z axis
bmesh.ops.mirror(bm, geom = bm.faces[:], axis='Z')

# Back to mesh
bm.to_mesh(mesh)

# Force update (if not the screen won't refresh immediatly)
mesh.update()